Thermodynamics of manganese oxides: Sodium, potassium, and calcium birnessite and cryptomelane Nancy Birknera,b,c and Alexandra Navrotskya,b,1 aPeter A. Rock Thermochemistry Laboratory, University of California, Davis, CA 95616; bNanomaterials in the Environment, Agriculture, and Technology, University of California, Davis, CA 95616; and cPratt School of Engineering, Duke University, Durham NC 27708 Contributed by Alexandra Navrotsky, December 21, 2016 (sent for review October 10, 2016; reviewed by R. Lee Penn and Jeffrey Post) Manganese oxides with layer and tunnel structures occur widely state are important for stabilizing these minerals and enhancing in nature and inspire technological applications. Having variable their functionality. compositions, these structures often are found as small particles There have been a number of qualitative observations of (nanophases). This study explores, using experimental thermo- poorly understood phenomena concerning phase formation i chemistry, the role of composition, oxidation state, structure, and/or transformation among birnessite and cryptomelane. ( ) and surface energy in the their thermodynamic stability. The mea- All natural and synthetic birnessite and cryptomelane phases sured surface energies of cryptomelane, sodium birnessite, potas- appear to have small particle size, less than 100 nm and often much smaller, at their initial formation. However, on the geo- sium birnessite and calcium birnessite are all significantly lower logic time scale, these natural minerals are able to grow in size than those of binary manganese oxides (Mn3O4,Mn2O3, and (coarsen) to form particles of larger dimensions, particularly MnO2), consistent with added stabilization of the layer and tunnel within the layers. Such growth often involves complex chemical structures at the nanoscale. Surface energies generally decrease reactions with changes in cation content, hydration, and oxida- with decreasing average manganese oxidation state. A stabilizing tion state. (ii) Mixed sodium–potassium birnessite often forms in enthalpy contribution arises from increasing counter-cation con- synthesis from aqueous solution, even when one is trying to make tent. The formation of cryptomelane from birnessite in contact the ion-exchanged potassium form. (iii) Synthesis of calcium with aqueous solution is favored by the removal of ions from the birnessite most successfully begins with sodium birnessite as the layered phase. At large surface area, surface-energy differences starting phase followed by exchange of calcium for sodium make cryptomelane formation thermodynamically less favorable under aqueous conditions. (iv) Cryptomelane exists initially 2 than birnessite formation. In contrast, at small to moderate sur- only within a limited size range, with surface area ∼50–350 m /g face areas, bulk thermodynamics and the energetics of the aque- (2, 11), when synthesized in the laboratory under ambient ous phase drive cryptomelane formation from birnessite, perhaps conditions. Again, given enough time and thermodynamic aided by oxidation-state differences. Transformation among birnes- driving forces, macroscale size may be achieved, especially in natural materials. (v) The synthesis of cryptomelane in an site phases of increasing surface area favors compositions with aqueous medium appears to begin with potassium birnessite lower surface energy. These quantitative thermodynamic find- formation and then requires leaching of potassium from the ings explain and support qualitative observations of phase- birnessite layers before spontaneous formation of the tunnel transformation patterns gathered from natural and synthetic structure (11). manganese oxides. In addition to kinetic controls on reaction rates, these observa- tions must have fundamental thermodynamic underpinning, and yet manganese oxides | birnessite | cryptomelane | calorimetry | the thermodynamic properties of these phases as a function of thermodynamics composition and particle size are poorly known. Some calorimetric studies of enthalpies of formation of layer and tunnel manganese omplex manganese oxides, highly reactive fine-grained ma- oxides have been reported (12), although their compositions in Cterials ubiquitous in nature, have served in a number of terms of AOS of manganese, and therefore oxygen content, were important capacities to benefit both the Earth and human society not measured and their particle sizes and surface areas were not ’ characterized. These factors have recently been shown to be im- (1). These oxides have profoundly affected Earth s critical zone portant in the thermodynamics of binary oxides in the Mn–Osystem throughout geologic time, influencing the evolution of the at- (13) and probably play an equal or more important role in the mosphere and the bioinorganic chemistry of organisms. They are complex layer and tunnel materials. effective in accumulation and recovery of economic ores and have recently inspired development of novel catalysts for green Significance chemistry and energy sustainability technologies. Minerals based on MnO include multiple classes, two of which are well known 2 Manganese oxides with layer and tunnel structures occur widely to geochemists: the 2 × 2 tunnel structure hollandite (the po- in the natural environment and inspire technological applications. tassium - bearing variety is cryptomelane) and the layered struc- In addition to having variable compositions, these structures often ture phyllomanganates (e.g., birnessite) (2, 3). Layered Mn oxides, are found as small particles (nanophases). This study explores, such as the mineral birnessite, derived from MnO2 by inclusion of using experimental thermochemistry, the role of composition, cations and water, with concomitant decrease in manganese av- oxidation state, structure, and surface energy in the thermody- erage oxidation state (Mn AOS) from 4 to typical values between namic stability of synthetic birnessites and cryptomelane. These 3.5 and 3.9, with manganese in tetravalent, trivalent and some- quantitative thermodynamic findings help explain and support times divalent oxidation states, are among the most important Mn qualitative observations of phase-transformation patterns gath- oxides in nature (4). These “nanosheet” Mn oxides readily trans- ered over the past five decades. form among phases, influencing crucial environmental and tech- nological processes, including biogeochemical cycles (1, 4, 5) water Author contributions: N.B. and A.N. designed research; N.B. performed research; N.B. and oxidation catalysis (6–8) and radionuclide confinement (9, 10). A A.N. analyzed data; and N.B. and A.N. wrote the paper. recent thermochemical study of layered calcium manganese ox- Reviewers: R.L.P., University of Minnesota; and J.P., Smithsonian Institution. ides (6) relevant to water oxidation catalysis (7, 8) suggests that The authors declare no conflict of interest. low surface energy and energetically accessible changes in oxidation 1To whom correspondence should be addressed. Email: [email protected]. E1046–E1053 | PNAS | Published online January 27, 2017 www.pnas.org/cgi/doi/10.1073/pnas.1620427114 Downloaded by guest on October 3, 2021 Table 1. Synthesis conditions for the samples PNAS PLUS Davies activity 2 2 Reagent, (i) [Ci]/M di z (z ) × (c) γ, Debye–Hückel coefficient Birnessite synthesis* + − H ; HCl 1.0 × e 2 0.9 1 0.01 0.717 1.421 Cl−; HCl 1.0 × e−2 0.3 1 0.01 0.470 1.421 Na+; NaCl 3.015 0.4 1 3.015 0.536 1.421 + K ; KCl 3.015 0.3 1 3.015 0.470 1.421 − Cl ; KCl 3.015 0.3 1 3.015 0.470 1.421 + Na ; NaMnO4 0.125 0.4 1 0.125 0.536 1.421 + K ; KMnO4 0.125 0.3 1 0.125 0.470 1.421 − MnO4 0.125 0.35 1 0.125 0.505 1.421 Cryptomelane synthesis† + H ;H2SO4 1.0 0.9 1 1 0.707 3.105 − HSO4 ;H2SO4 1.0 0.4 4 4 0.067 3.105 + K ; KCl 3.0 0.3 1 3 0.435 92.933 Cl−; KCl 3.0 0.3 1 3 0.435 3.105 + K ; KMnO4 0.125 0.3 1 0.13 0.435 3.105 − MnO4 ; KMnO4 0.125 0.35 1 0.13 0.475 3.105 Reagent parameters to calculate total ionic strength include: concentration [Ci] in mol/L of the reagent ions (i) with size parameter (di) and ionic charge (z). Debye–Hückel activity coefficient (γ) is computed from the Debye–Hückel equation, and the Davies activity coefficient is computed from Davies equation. *Final ionic strength: 1/2 Σz2c = 3.150. † Final ionic strength: 1/2 Σz2c = 5.625. The present work concentrates on measurements of en- The measured enthalpy associated with dropping a sample CHEMISTRY thalpies of formation and surface energies of synthetic birnessite from room temperature into molten oxide solvent at high tem- and cryptomelane phases to explain these frequently observed perature, where it dissolves, is called the enthalpy of drop solu- phase formation and transformation phenomena on the basis that tion, ΔHds. The difference between ΔHds of reactants and they are thermodynamically driven. Because the initially formed products (of the same composition) gives the enthalpy of re- materials are fine grained, indeed nanophase, surface energies action. Thus, the enthalpy associated with an increase in surface Δ play a large role in selective stabilization of different phases and area can be obtained by comparing Hds for samples of different are emphasized in this study. surface areas. Additionally, because the nanophase samples have different water contents, ΔHds is corrected to account for their Results surface-associated water, yielding a corrected drop solution en- Δ n Table 1 summarizes the synthesis parameters of the samples. thalpy, Hds,corr. The correction